This invention relates to a circuit for controlling the level of a color video signal for allowing a television video signal to be A/D converted on a liquid crystal color television receiver.
Recently the so-called liquid crystal television receivers have been implemented with a liquid crystal display panel on a display unit. The liquid crystal television receiver converts video signals amplified at video amplifiers to digital signals by virtue of A/D converters and drives the liquid crystal display panel by the digital signal to permit images to be displayed on the liquid crystal display device. In the liquid crystal display panel, however, it has been difficult to obtain an image of better contrast due to a narrower gradation range from a white to a black level as considered from the characteristics.
A solution to this problem is made by setting an average potential level of video signals constant with the use of a high-pass filter and setting the upper and lower limit reference voltages of an A/D converter through a resistor division whereby it is possible to obtain a better constant.
FIG. 1 shows one from of an A/D converter in a conventional liquid crystal color television receiver. The A/D converter includes color demodulator 1, contrast control volume 2 connected to color demodulator 1, A/D converters 3, 4 and 5 of a parallel comparison type connected to color demodulator 1 and reference voltage generators 6, 7 and 8.
A composite color video signal is input to color demodulator 1 to produce primary color signals R, G and B. The primary color signals R, G and B are supplied to reference voltage generators 6 to 8 to generate upper and lower limit reference voltages V.sub.H and V.sub.L. In this way, a bias is imparted to the video signal. A/D converters 3 to 5 convert the input video signals to digital signals DR1 to DR4, DG1 to DG4 and DB1 to DB4, respectively, in synchronism with a sampling clock so that they are delivered to liquid crystal drivers.
Since, however, the primary color signals R, G and B are supplied to reference voltage generators 6, 7 and 8, respectively, then corresponding different reference voltages are generated for A/D conversion. That is, the primary signals R, G and B are A/D converted based on the respective different voltages. When an image is to be displayed on the liquid crystal display unit on the basis of the digital signals after the A/D conversion has been achieved, the R:G:B ratio will be deviated from that of the original primary color signals R, G and B, so that these colors cannot faithfully be reproduced due to a consequent variation in the tone of colors.